382 ON LIGHT. 



especially is seen which would lead to a suspicion that 

 the crystal were other than an ordinary piece of glass. 

 But if, in this state of things, between the crystal and 

 the eye, be placed another tourmaline plate, having its 

 axis horizontal, a magnificent set of coloured rings will be 

 seen ; the exact counterpart of the reflected rings described 

 by Newton (only infinitely more vivid and brilliant), in 

 every respect but these : First, that they are all divided 

 into four quadrants of coloured light by a dark cross 

 passing through their common centre, and having its 

 arms vertical and horizontal; and, secondly, that the 

 rings themselves are of unequal brightness in different 

 parts of their circumference, being most luminous at the 

 middle points of the quadrants into which the cross 

 divides them, and fading away very gradually on either 

 side of these points, till they cease to be traceable and 

 are lost in the darkness of the cross. On the other 

 hand, if the tourmaline plate between the eye and the 

 crystal (which we shall call the " analyzing plate" or the 

 " analyzer," for a reason which will presently appear) be 

 placed with its axis vertical, a series of rings will also be 

 seen : but they are, now, the complementary series 

 those seen by transmission in the Newtonian experi- 

 ment ; and the cross, instead of black, is now white. 

 Lastly, if the analyzing plate be placed obliquely, both 

 sets of rings will be partially, and, as it were, confusedly, 

 exhibited ; the one dislocating the other, in consequence 

 of the brighter annuli of the one set abutting upon the 

 obscurer of the other, the reds on the greens, the purples 

 on the yellows, &c. : the preponderance in light, distinct- 



